1. Field of the Invention
The present invention relates to a display device which consumes less electric power, and which has high screen brightness. In particular, the present invention relates to an improvement of a display device for displaying a picture image corresponding to an image signal on an optical waveguide plate by controlling leakage light at a predetermined position on the optical waveguide plate by controlling the displacement action of an actuator element in a direction to make contact or separation with respect to the optical waveguide plate in accordance with the attribute of the image signal to be inputted.
2. Description of the Related Art
Known display devices include, for example, cathode ray tubes (CRT) and liquid crystal display devices.
Cathode ray tubes include, for example, ordinary television receivers and monitor units for computers. Although the cathode ray tube has a bright screen, it consumes a large amount of electric power. Further, the cathode ray tube involves a problem that the depth of the entire display device is large compared with the size of the screen.
On the other hand, the liquid crystal display device is advantageous in that the entire device can be miniaturized, and the display device consumes a small amount of electric power. However, the liquid crystal display device is inferior in brightness of the screen, and the field angle of the screen is narrow.
In the case of the cathode ray tube and the liquid crystal display device, it is necessary for a color screen to use a number of picture elements (image pixels) which is three times a number of picture elements used in a black-and-white screen. For this reason, the device itself is complicated, a great deal of electric power is consumed, thereby inevitably resulting in increased cost.
In order to solve the problems described above, the present applicant has suggested a novel display device (see, for example, Japanese Laid-Open Patent Publication No. 7-287176). As shown in FIG. 20, this display device includes actuator elements 200 arranged for respective picture elements. Each of the actuator elements 200 comprises a main actuator element 208 including a piezoelectric/electrostrictive layer 202 and an upper electrode 204 and a lower electrode 206 formed on upper and lower surfaces of the piezoelectric/electrostrictive layer 202 respectively, and a substrate 214 including a vibrating section 210 and a fixed section 212 disposed under the main actuator element 208. The lower electrode 206 of the main actuator element 208 contacts with the vibrating section 210. The main actuator element 208 is supported by the vibrating section 210.
The substrate 214 is composed of a ceramic in which the vibrating section 210 and the fixed section 212 are integrated into one unit. A recess 216 is formed in the substrate 214 so that the vibrating section 210 is thin-walled.
A displacement-transmitting section 220 for obtaining a predetermined contact area with an optical waveguide plate 218 is connected with the upper electrode 204 of the main actuator element 208. In the illustrative display device shown in FIG. 20, the displacement-transmitting section 220 is arranged such that it is located closely near to the optical waveguide plate 218 in the OFF selection state or in the NO selection state in which the actuator element 200 stands still, while it contacts with the optical waveguide plate 218 in the ON selection state at a distance of not more than the wavelength of the light.
The light 222 is introduced, for example, from a lateral end of the optical waveguide plate 218. In this arrangement, all of the light 222 is totally reflected at the inside of the optical waveguide plate 218 without being transmitted through front and back surfaces thereof by controlling the magnitude of the refractive index of the optical waveguide plate 218. In this state, a voltage signal corresponding to an attribute of an image signal is selectively applied to the actuator element 200 by the aid of the upper electrode 204 and the lower electrode 206 so that the actuator element 200 is allowed to perform various displacement actions of ON selection, OFF selection, and NO selection. Thus, the displacement-transmitting section 220 is controlled for its contact and separation with respect to the optical waveguide plate 218. Accordingly, the scattered light (leakage light) 224 is controlled at a predetermined portion of the optical waveguide plate 218, and a picture image corresponding to the image signal is displayed on the optical waveguide plate 218.
When a color picture is displayed by using the display device, the following operation is performed. For example, light sources for three primary colors are switched to control the light emission time for three primary colors, while synchronizing the contact time between the optical waveguide plate and the displacement-transmitting plate with the period of color development. Alternatively, the contact time between the optical waveguide plate and the displacement-transmitting plate is controlled, while synchronizing the light emission time for three primary colors with the color development period.
Therefore, the illustrative display device suggested by the present applicant is advantageous in that it is unnecessary to increase the number of picture elements as compared with the black-and-white screen, even when the display device is applied to the color display system.
However, in the case of the illustrative display device suggested by the present applicant, it is necessary to display three images in total, i.e., an image for R, an image for G, and an image for B, within a prescribed one field period. Therefore, the horizontal frequency should be made extremely quick.
For this reason, a problem arises in that the electric power consumption is increased, and the peak current is also increased. Further, an inconvenience occurs in that the cost becomes highly expensive, because it is required to use an actuator element having a quick response speed, and the driving circuit arrangement is also complicated.